This invention relates to waveform correction in modulated focus voltage circuits for television apparatus.
The scanning distance of cathode ray tubes from the center of deflection to the scanned raster is not uniform. The scanning distance varies markedly as the beam is deflected horizontally and vertically from the central portion of the screen, and consequently, defocusing is most severe at the corners of the raster. The adoption of picture tubes of increased size and wider deflection angles in recent years has emphasized the defocusing effect at the corners of the scanning pattern.
Efforts have been made to control focus in electrostatically focused tubes by changing the magnitude of the focus potential with displacement of the beam from the center of the raster, that is, by dynamic focusing. A unidirectional potential to the focus electrode of the electrostatically focused cathode ray tube establishes the proper focus condition at the center of the raster and the concurrent application of a suitable varying potential maintains that condition throughout the scanning raster.
In the usual case, the beam focus changes approximately in accordance with a parabolic function along either scanning direction. Accordingly, the varying components of focus potential may have a parabolic waveform in both axes. Typically, a vertical rate parabolic waveform is produced by integrating a sawtooth waveform synchronized, for example, to a vertical deflection current. Multiplication can also be used. However, both techniques may require complex circuitry.
A vertical rate trapezoidal waveform can be produced from a vertical sawtooth signal using a piece wise linearization technique. The trapezoidal waveform is applied to the focus electrode and approximates the aforementioned parabolic waveform. The circuitry may be less expensive and better controlled because no signal integration is required. Such a dynamic focus voltage generating apparatus may include a source of a first signal at a frequency related to a deflection frequency. A switching arrangement is responsive to the first signal for producing piece wise linearized portions of a modulation signal, during corresponding portions of a trace interval of a deflection cycle. The focus voltage is modulated in accordance with the modulation signal.
In such a dynamic focus circuit such as described above, there can exist a time compressed undesired copy of the desired trace waveform during vertical retrace. This waveform occurs during vertical blanking and therefore does not improve the focus of the picture. On the contrary, it causes an extra unneeded variation in the focus voltage than can interact with other circuits such as high voltage regulation such that undesired ringing occurs.